Kali, a male polar bear that was orphaned in Point Lay, Alaska, explores the enclosure outside the infirmary at the Alaska Zoo on March 22, 2013, in Anchorage. / Bob Hallinen, AP

by Jolie Lee, USA TODAY Network

by Jolie Lee, USA TODAY Network

You probably already know that the world's polar bear population is threatened due to a warming planet and melting ice.

But how is climate change affecting polar bears now?

USA TODAY Network spoke with Steven Amstrup, chief scientist with Polar Bears International, a polar bear conservation group, about how far-reaching the problem is today. Armstrup warns that the threats to polar bears are "harbingers of what's to come for all of us."

1. Disappearing population

Polar bears are considered a threatened species in the USA. An estimated 20,000-25,000 polar bears remain worldwide, but Amstrup predicts two-thirds of the population will be gone by midcentury.

Polar bears feed on seals that they hunt on sea ice. Seals are rarely on land, and it's even more rare for polar bears to catch seals in the water.

"Regardless of how many seals there might be, polar bears don't have access to them without the sea ice," Amstrup said.

Since 1979, Arctic sea ice in September has been declining at a rate of about 13% per decade, according to the National Snow and Ice Data Center.

The melting ice correlates to the planet's rising temperature over the past century, according to NASA measurements of global ocean-land temperatures.

September is when the Arctic sea ice is at its lowest size. Historically, over most of their range, polar bears hunt on the ice through the summer, fall and into the winter.

2. Cannibalism? Yes, cannibalism.

Researchers think the longer ice-free seasons that have prevented polar bears from getting to seals are forcing them to find other sources of nutrition - each other.

All bears will eat other bears in certain circumstances, including for population regulation, reproductive advantage or establishing dominance, according to a 2006 study Amstrup published in the journal Polar Biology.

But in recent years, researchers observed behavior they'd never seen before: polar bears "stalking, killing and eating" other polar bears, according to the study.

Amstrup and fellow scientists recorded three separate incidents of cannibalism in Alaska and Canada between January and April 2004.

"We can't say for sure that this is the fingerprint of climate change, but it's consistent with the kinds of things you expect to see when a population is nutritionally stressed," Amstrup said.

3. More contact with humans

To find food, polar bears are traveling to places they don't normally go. Specifically, they're showing up in villages, which has happened in northern Canada, Amstrup said.

Because of these increased sightings, some people may wrongly think the polar bear population is growing, Amstrup said.

The fact is, their population overall is shrinking, he said.

"A polar bear is not going to lay down and die when it's hungry," Amstrup said. What we're seeing now is bears "becoming very common where (before) they were uncommonly seen."